Municipal sewage sludge is currently disposed of by dumping into a landfill or the ocean or by incinerating it. Land-filling has become more expensive as dump sites close to urban centers are filled to their capacity. In addition, some sewage sludges contain enough heavy metals to render them hazardous wastes, although sewage sludge is specifically exempted from certain current regulations regarding wastes with heavy metals. Ocean dumping is increasingly considered ecologically unsound and subject to regulation or prohibition. Incineration is expensive due to fuel consumption and also produces an ash product which is itself hazardous and includes even higher concentrations of heavy metals.
In addition to outright disposal, consideration has been given to the use of sewage sludge to produce a useful product such as fertilizer. One such product that has been in use for many years is produced by drying the sludge and merely using the dried sludge as fertilizer without further treatment. An example of this type of fertilizer product is that known as Milorganite, a trademark of Metropolitan Sewerage District of the County of Milwaukee. For efficient drying, temperatures of at least about 70.degree. C. to 100.degree. C. are employed, significantly reducing the available nitrogen in the material.
Sewage sludge, as collected and without treatment, includes, principally water with a minor amount of solids containing nitrogen compounds. The solids include human or animal waste containing ammonia; other nitrogen-containing material may also be included. In a typical municipal sewage sludge (raw settled sludge), untreated, the amount of water is typically about 95%, by weight. Of the solids contained in the sludge there is an amount of available nitrogen, primarily present as ammonia, typically of about 2% to 5%, by weight, of the weight of the solids, before drying. After drying, at temperatures of at least about 100.degree. C., the sludge contains a much higher solids content, of course, but the available nitrogen content has been substantially reduced. This reduction in nitrogen is primarily due to the thermal decomposition of ammonia and the evolution of nitrogen and NO.sub.X.
The disadvantages of the conventional process for converting sewage sludge to fertilizer include:
A low nitrogen content of the product;
High cost for the drying process in relation to the low nitrogen content of the product;
A product required to be used in large volume for effective fertilizing with attendant costs of transport;
A product in which heavy metal concentrations may be high.
By way of further background, copending application Ser. No. 07/209,675, filed Jun. 21, 1988, by C. W. Mallory and assigned to the present assignee, deals with a method of treating dewatered wastes, such as sewage sludge, in order to render such wastes environmentally acceptable for ultimate disposal. The method of the copending application involves drying the waste, converting the dried waste to granular solids through agglomeration, and adding alkaline metal silicate and a solidification agent such as cement to produce a product in granular form in which metals and other toxic materials have become chemically fixed.
The purposes of the invention include providing a method for avoiding loss of available nitrogen in the drying of sewage sludge while producing a dried product suitable for agricultural fertilizer use. An additional object is to provide an improved process for sewage sludge disposition that alleviates former economic disadvantages and environmental concerns.
In accordance with the present invention a process is carried out in which a starting material, such as municipal sewage sludge, is used which includes principally water with a minor amount of solids containing nitrogen compounds. The starting material is treated before exposure to elevated temperatures by adding an amount of an acid sufficient to produce a pH in the treated material of about 7 or less. Then water is thermally removed from the treated material.
The acid treatment places the nitrogen containing ammonia of the sludge into thermally stable compounds. For example, using nitric acid or phosphoric acid, ammonium nitrate or diammonium phosphate, respectively, are produced. The amount of acid required is not expensive and thermal drying temperatures of at least about 100.degree. C., in excess of the boiling point of ammonium hydroxide in which the unfixed ammonia tends to occur, can be employed for drying and still yield a dried product usable as a fertilizer with a nitrogen content containing about 5% nitrogen as ammonia.
Other aspects of the invention will become more apparent from the ensuing description.